Chapter 18: Network Security pot

William Stallings
Data and Computer
Communications
Chapter 18
Network Security

Security Requirements

Confidentiality

Integrity

Availability

Passive Attacks

Eavesdropping on transmissions

To obtain information

Release of message contents

Outsider learns content of transmission

Traffic analysis

By monitoring frequency and length of messages,
even encrypted, nature of communication may be
guessed


Difficult to detect

Can be prevented

Active Attacks

Masquerade

Pretending to be a different entity

Replay

Modification of messages

Denial of service

Easy to detect

Detection may lead to deterrent

Hard to prevent

Security Threats

Conventional Encryption

Ingredients

Plain text


Encryption algorithm

Secret key

Cipher text

Decryption algorithm

Requirements for Security

Strong encryption algorithm

Even if known, should not be able to decrypt or work
out key

Even if a number of cipher texts are available
together with plain texts of them

Sender and receiver must obtain secret key
securely

Once key is known, all communication using this
key is readable

Attacking Encryption

Crypt analysis

Relay on nature of algorithm plus some knowledge of
general characteristics of plain text


Attempt to deduce plain text or key

Brute force

Try every possible key until plain text is achieved

Algorithms

Block cipher

Process plain text in fixed block sizes producing block
of cipher text of equal size

Data encryption standard (DES)

Triple DES (TDES)

Data Encryption Standard

US standard

64 bit plain text blocks

56 bit key

DES
Encryption
Algorithm

DES Single
Iteration

Strength of DES

Declared insecure in 1998

Electronic Frontier Foundation

DES Cracker machine

DES now worthless

Alternatives include TDEA

Triple DEA

ANSI X9.17 (1985)

Incorporated in DEA standard 1999

Uses 3 keys and 3 executions of DEA algorithm

Effective key length 168 bit

Location of Encryption Devices

Link Encryption

Each communication link equipped at both ends


All traffic secure

High level of security

Requires lots of encryption devices

Message must be decrypted at each switch to
read address (virtual circuit number)

Security vulnerable at switches

Particularly on public switched network

End to End Encryption

Encryption done at ends of system

Data in encrypted form crosses network
unaltered

Destination shares key with source to decrypt

Host can only encrypt user data

Otherwise switching nodes could not read header or
route packet

Traffic pattern not secure


Use both link and end to end

Key Distribution

Key selected by A and delivered to B

Third party selects key and delivers to A and B

Use old key to encrypt and transmit new key
from A to B

Use old key to transmit new key from third party
to A and B

Automatic Key Distribution
(diag)

Automatic Key Distribution

Session Key

Used for duration of one logical connection

Destroyed at end of session

Used for user data

Permanent key

Used for distribution of keys


Key distribution center

Determines which systems may communicate

Provides one session key for that connection

Front end processor

Performs end to end encryption

Obtains keys for host

Traffic Padding

Produce cipher text continuously

If no plain text to encode, send random data

Make traffic analysis impossible

Message Authentication

Protection against active attacks

Falsification of data

Eavesdropping

Message is authentic if it is genuine and comes

from the alleged source

Authentication allows receiver to verify that
message is authentic

Message has not altered

Message is from authentic source

Message timeline

Authentication Using
Encryption

Assumes sender and receiver are only entities
that know key

Message includes:

error detection code

sequence number

time stamp

Authentication Without
Encryption

Authentication tag generated and appended to
each message


Message not encrypted

Useful for:

Messages broadcast to multiple destinations

Have one destination responsible for authentication

One side heavily loaded

Encryption adds to workload

Can authenticate random messages

Programs authenticated without encryption can be
executed without decoding